Picorna and Flaviviruses encompass a large variety of medically important human viruses which include those inducing poliomyelitis (poliovirus), infectious (Hep. A) and chronic hepatitis (hepatitis C), common cold (rhinovirus), and myocarditis and encephalitis (coxsackie) among others. A common feature of these viruses is the strategy they employ for synthesis of viral proteins. While cellular mRNAs are translated by cap-dependent "scanning" mechanism, the viral RNAs are translated by a distinct mechanism involving internal entry of ribosomes within the 5' untranslated region (5' UTR) of viral RNA (IRES-mediated translation). Two inhibitors (a small RNA, IRNA and a small peptide, LAP) which efficiently block hepatitis C and poliovirus IRES-mediated translation (but not cellular translation) both in vivo and in vitro, will be studied. Both biochemical and genetic approaches will be used to determine the mechanism by which IRNA and LAP preferentially inhibit translation of HCV and PV RNAs over cellular mRNAs. Cellular proteins involved in IRES-mediated translation will be purified and their roles in HCV RNA translation will be determined. Structure-function analysis of IRNA will be performed. The mechanism of entry of LAP into hepatocytes will be determined. Identity and normal function of IRNA in the yeast S. cerevisiae will be addressed by cloning and characterizing the IRNA gene. IRNA gene knockout will be performed to better understand the role of IRNA in yeast. We will examine whether IRNA-binding proteins in yeast play any role in IRES-mediated translation (in yeast). Finally, we will develop transgenic mice expressing IRNA and LAP to determine the long term expression of IRNA in animals as well as efficacy of IRNA in blocking virus infections in transgenic animals. It is hoped that the studies proposed here will not only further our understanding of the mechanism of internal initiation of translation in eukaryotic cells, but also provide novel strategies to develop antivirals effective against hepatitis C.